The craftsman's monocle can be obtained after you've started the second part of the miniquest that Darren Lightfinger has to offer.

To obtain this item, talk to Urist Loric after the first caper, From Tiny Acorns. Choose the dialogue option, "Is that your talisman by your foot there?" He will bend down and you will automatically steal his Craftsman's monocle. The monocle is useful to those crafting clockwork items on their POH workbench, allowing such items to be made 2 levels earlier than usual. It has been noted that it might change the hair style of your chat head if you have the medium or fringe hair style.

There are no requirements to wear it, other than the 24 Thieving needed to start the caper.